Non-ultraviolet photocatalytic kinetics of NaYF4:Yb,Tm@TiO2/Ag core@comby shell nanostructures

Abstract

An effective near-infrared (NIR) active photocatalyst, NaYF₄:Yb,Tm@TiO₂/Ag (UC@TiO₂/Ag) core@comby shell composite, was synthesized by a simple three-step hydrothermal process. Under the full-spectrum light of a Xe lamp in UV-Vis absorbance experiments, about 96% of R6G dyes in solution were degraded by UC@TiO₂/Ag in 120 minutes, while only about 64% of the dyes were degraded by pure TiO₂ under the same conditions. Under a UV-filtered Xe lamp, about 35% of the dyes were degraded by UC@TiO₂/Ag in 120 minutes; interestingly, only about 8% of the dyes were degraded by pure TiO₂. Under irradiation by a 785 nm laser in surface enhanced Raman scattering (SERS) experiments, the photodegradation rate constants were 0.02612 s⁻¹ for UC@TiO₂/Ag and 0.00046 s⁻¹ for TiO₂/Ag, indicating a nearly 58 fold improvement. After deducting the photobleaching effect, the photodegradation rate constants for UC@TiO₂/Ag under 633 and 532 nm lasers were 0.00715 s⁻¹ and 0.00565 s⁻¹, respectively, revealing a sharp decrease under irradiation at shorter wavelengths. Electron spin resonance (ESR) analysis revealed that the presence of UC in this photocatalytic system certainly induced the increase of ˙OH free radicals with NIR irradiation, i.e. the UC core converts NIR light into ultraviolet (UV) light and initiates excellent photocatalytic activity of the TiO₂/Ag comby shell. Furthermore, the decorating of Ag nanoparticles not only enhances the photocatalytic ability, but also provides a structural basis for monitoring the photocatalytic kinetics by the SERS technique. By virtue of monochrome laser lines, SERS analysis provides direct evidence to prove the capability of UC-initiated non-UV photocatalysis and the improvement of the utilization of non-UV lights on TiO₂. The results revealed that this new photocatalytic platform can efficiently utilize different bands of the solar spectrum and also find new applications in SERS fields.